AN EXPERIMENTAL TEST OF ALTERNATIVE EQUILIBRIA: THE DAPHNIA-CYANOBACTERIA INTERACTION IN NATURE

The Daphnia-cyanobacteria interaction may lead to alternative equilibria in eutrophic lakes. Bloom-forming bluegreens are relatively resistant to grazing and may negatively effect Daphnia population growth. At the same time, high densities of Daphnia can maintain low phytoplankton abundance and prevent succession to bluegreens. Taken together, these observations suggest that alternative equilibria (high Daphnia, low bluegreens versus low Daphnia, high bluegreens) may exist depending on whether Daphnia or bluegreens attain dominance first.
To test this idea, I established fish and no-fish enclosures within a eutrophic lake. After ~70 days, enclosures with fish had few Daphnia and dense populations of bluegreens (Microcystis, Anabaena), while no-fish enclosures had dense populations of Daphnia and very low phytoplankton. Fish were then removed from some enclosures to assess whether Daphnia would rebound and graze down the bluegreen-dominated phytoplankton assemblage. After 30 days, fish-removal enclosures became extremely clear as Daphnia returned, while enclosures with fish remained green, indicating that the bluegreen-dominated state was invasible. Although more resistant to invasion, the Daphnia-dominated state was not completely stable as enclosures that never had fish began to resemble enclosures with fish after 90 days.